Pandoc can already convert from epub to markdown and we have a markdown to 30040/30041 converter/uploader that we're already using to publish books. You can already convert all the books. Would be nice to have a pretty online website for doing it, I agree, but that's not really what we're discussing.

We're discussing a nostr-Native e-reading app that imports and exports ePubs (or any other format, pandoc does all of the major ones), but uses notes stored on relays internally. 

 You can try out Pandoc here:
https://pandoc.org/demos.html